Perhaps they have a rivalry with another group of characters. For example, they are competing in a grand tournament where they have to show off their unique skills. This could involve combat skills, magic abilities, or strategic thinking. And as the story progresses, they learn more about themselves and their relationship with each other.
A really interesting plot could be that the Bowsette sisters get transported to a different dimension. In this new world, they have to adapt to different rules and cultures. They meet new friends and foes alike. While trying to find their way back home, they end up changing the fate of this new dimension in unexpected ways. This would allow for a lot of character development and exploration of different settings.
One idea could be that the Bowsette sisters go on an adventure to find a magical artifact that can save their kingdom from a dark force. They face various challenges along the way, like tricky puzzles and powerful enemies.
Another plot could be that Bowsette accidentally casts a spell that makes Mario forget all about his previous adventures and relationships. Bowsette then has to re - introduce herself to Mario and try to make him fall in love with her from scratch. This would involve her showing him the beauty and magic of the Mushroom Kingdom in a new light, while also facing challenges from other characters who are suspicious of her motives.
Another plot idea could be a what - if scenario. For example, what if one of the sisters had a different power? This could change the whole dynamic between them and the story. Say, if Elsa had the power to control time instead of ice. It would lead to different challenges and interactions with Anna, like having to deal with the consequences of changing time to save someone, but also the potential chaos it could cause in Arendelle.
Obviously, the Bowsette sisters themselves are the main characters. They are likely to have different personalities. One might be more outgoing and brave, while the other could be more reserved and intelligent.
Some fanfic ideas might include a story where one of the siblings gets into trouble and the others have to come to their rescue. It could be legal trouble, like being wrongly accused of something. Or it could be about a relationship problem that one is having, and the siblings use their own experiences and wisdom to help them through it.
One idea could be exploring the backstories of the sisters more in - depth. Maybe each sister has a secret talent or a past trauma that hasn't been fully revealed in the original show. Another idea is to create an alternate universe where the sisters have different occupations but still maintain their strong bond. For example, one could be a famous artist, another a scientist, and they have to deal with the challenges of their new lives while still being there for each other.
A plot idea could be that the Sisters Grimm discover a hidden realm within their own world that has all the forgotten or lost fairy - tales. They then have to figure out how this realm is connected to the problems in their own town. Also, a story where they swap powers or abilities for a day and have to deal with the chaos that ensues. For example, if Sabrina has Daphne's ability to see the good in people and Daphne has Sabrina's more cynical view, it would lead to some interesting character development.
A plot about the sisters getting superpowers could be really exciting. They could start off not knowing how to control their powers and have some comical and also dangerous situations as a result. For example, one sister might accidentally set off her power in public and they have to figure out how to cover it up while also learning to use it properly.
There are fanfictions where Bowsette x Bowsette focuses on their relationship development. It could start with them being rivals at first, constantly competing for something, like the affection of a certain character. But as the story progresses, they start to understand each other better and form a strong bond. They might also discover hidden secrets about their pasts that bring them closer.
A plot could revolve around Harry having to team up with the Black sisters to fight a new dark force in the wizarding world. The sisters have their own motives and powers, and Harry has to navigate their complex personalities while they battle the enemy. For example, they might need to break into an ancient, cursed vault together, relying on each other's skills.
One idea could be that they discover a long - lost family secret together. Maybe there's a hidden inheritance or a special power that runs in the family.